At present, based on the Blu-ray Disc™ standard using a blue-violet semiconductor laser, the commercialization of an optical disc having about the 50-GB recording capacity is also becoming increasingly available in its consumer-product use. From now on, it is also wished in the optical disc to accomplish its larger-capacity implementation up to about the HDD (Hard Disk Drive) recording capacity, i.e., 100 GB through 1 TB.
In order to implement an ultra-high recording density like this by using the optical disc, however, it is required to provide a high-density implementation technology which is based on a new scheme. Here, this new scheme is needed to be different from the high-density implementation technology based on the shorter-wavelength implementation and objective lens' higher NA implementation.
In the circumstances where the studies concerning next-generation storage technologies are underway, attention is focused on a hologram recording technology for recording digital information by using holography.
The hologram recording technology is the following technology: A signal light, which has page-data information modulated in a two-dimensional manner by a spatial optical modulator, is superimposed on a reference light within a recording medium. Then, a refractive-index modulation is caused to occur within the recording medium by an interference-fringe pattern produced at the time of this superimposition, thereby recording the information into the recording medium.
At the time of reproducing the information, the reference light used at the information-recording time is irradiated onto the recording medium. As a result of this irradiation, the hologram recorded within the recording medium operates as if it is a diffraction grating, thereby generating a diffraction light. This diffraction light is reproduced as the identical light, including the signal light and phase information recorded.
The signal light reproduced is detected in a two-dimensional manner and at a high speed, using an optical detector such as CMOS or CCD. In this way, the hologram recording technology allows two-dimensional information to be recorded into an optical recording medium at a stroke by using a single hologram. Moreover, this technology allows this information to be reproduced. Furthermore, this technology makes it possible to overwrite plural pieces of page data at a location where the recording medium exists. These features make it possible to accomplish the large-capacity and high-speed recording/reproduction of information.
As the configuration of a hologram recording/reproducing apparatus, the description is given in PATENT LITERATURE 1, for example. In the present document, the following description is given: “The shape of a recording medium 1 is a disk-like shape, for example, and this recording medium 1 is fixed onto a spindle motor 200 by a clamp mechanism. The rotational driving of the spindle motor 200 is so designed as to allow the irradiation position of a coherent beam on the recording medium to be displaced in the tangential direction. The spindle motor 200 is fixed onto a thread motor 201. The rotational-transportation driving of the thread motor 201 is so designed as to allow the irradiation position of the coherent beam on the recording medium to be displaced in the radial direction as well.”